Ok, so sometimes I come up with a question while I’m working…but other times I just see something here on LJ that surprises the heck out of me and want to know more.

This is one of the later category. It is also admittedly “splitting hairs” and rather low in relevance. Kind of a snarky post upon re-reading. Please look past that. I hope to learn something from the responses.

On another thread, somebody mentioned using “600 grit” sandpaper between coats of waterbased poly.

Ok, fine. Normally I think of “220 grit” when I think “so fine it is hardly worth the trouble”. So 600 kind of bowled me over.

I googled and discovered that some Germans are actually making money selling 7000 grit sandpaper!

I would think just plain old copier paper would have a “grit” rating of maybe 1000. What’s the deal?

I looked up grit size on wikipedia and found it is “almost, but not rigorously linear”. That is to say it is certainly not a logarithmic progression or anything of the sort. It is also not directly related to the size of the particles by any obvious formula. (example: 100 grit has particles of size 140 micrometers, and 1000 is NOT 14, but rather 10.3)

Anyway, 600 grit for between coats? Seriously? Why not just rub your calloused hand over it? Or rub it on your t-shirt?

What is the stated goal of that between coats sanding, and is it really better served with such a high grit? Would more gentle use of 220 grit accomplish the same thing? Can one of you provide me with an “explain it to me like I’m six” sort of explanation. “You are trying to remove all of the———- without disturbing the—————.”

The stated size of the grit particles for 220 is 68 micrometers. Both that and 600 grit (25 micrometers) fall within the alleged range of the thicknesses of human hair (17 for a georgia peach face hair to 181 for an RCH).

Does it make a difference? I have gently used some steel wool for between the coats “sanding”, and it seemed like it went fine. Not sure I would have known the difference if I had skipped that step. I am wondering if my senses are just not finely tuned enough to know the difference.

-- Last week I finally got my $*i# together. Unfortunately, it was in my shop, so I will probably never find it again.

17 replies so far

I knock down my lacquer between the 3rd and 4th coats with 3 to 400 grit. Between the last coat and the wax with 600. It does make a difference. As the lacquer is applied it goes on as droplets. The 300 merely knocks the tops of these droplet off. 220 gets below the tops of the droplets and hits the base, if you will of the finish. The next two coats, just a bit of build up, come out smoother than the first three. The 600 again just knocks the tops of the droplet off until it ‘feels’ silky. The wax which is my last coat finish fills any scratches the 600 leave and any droplets that I missed.

I usually do 320 or 400 between coats, or between every other coat, depending on the application, only for the purpose of knocking down any dust nibs before the next coat. This is with very light pressure. I personally think 220 is way to abrasive for this purpose. Maybe they both fall in the range for the thickness of a human hair, but that’s a pretty big range. Feel 220, and feel 600, and its pretty clear that they’re world’s apart in terms of abrasiveness.

Common tire pressure range for passenger vehicles and light trucks is usually in the 30-40 range. That doesn’t mean I can run my truck’s tires at any pressure in that range and expect the results to be negligible.

-- - The mightiest oak in the forest is just a little nut that held its ground.

If you need a good visual of the difference, take a piece of dark wood like black walnut and finish it with a hard finish like shellac or laquer. After it has cured, sand part half of the finished board with 220 and the other half with 600. You will see a difference.

That said, most film finishes will benefit with light sanding between coats with 320-400 grit. I never use 600 unless it is before the last coat of a finish I will be rubbing out. Some dense woods (Ipe) will get sanded to 1500 grit, but that’s for an oil and wax finish. I would not go beyond 600 grit for a film finish.

I think of “220 grit” when I think “so fine it is hardly worth the trouble”. ...I would think just plain old copier paper would have a “grit” rating of maybe 1000. What s the deal?- JeffP

Nah, you’re way off base, not even in the same ballpark as the base, probably not in the same town as the ballpark. 800 – 1000 will give you a satin finish on metal, you need 2000 minimum for a mirror finish. 220 is fine on bare wood because wood has a structure not unlike a bundle of straws. When the scratches are going with the grain they are not noticeable. If you want to see how coarse 220 is, sand cross grain or sand a film finish.

Same here. 220 to rough for the poly and lacquer I use unless I have to knock down some big runs with a card scraper. Then I just work my way up to 600. 600 is the top end of the finest grit I’m willing to use unless wet sanding. Which I’ve never done. Yet.

-- Shooting down the walls of heartache. Bang bang. I am. The warrior.

I think of “220 grit” when I think “so fine it is hardly worth the trouble”. ...I would think just plain old copier paper would have a “grit” rating of maybe 1000. What s the deal?- JeffP

Nah, you re way off base, not even in the same ballpark as the base, probably not in the same town as the ballpark. 800 – 1000 will give you a satin finish on metal, you need 2000 minimum for a mirror finish. 220 is fine on bare wood because wood has a structure not unlike a bundle of straws. When the scratches are going with the grain they are not noticeable. If you want to see how coarse 220 is, sand cross grain or sand a film finish.

- Rick M.

So maybe this is it…I was thinking sanding == smoothing. But here when using it between finish coats we’re more talking about “polishing” rather than smoothing? I’m used to thinking of sanding in terms of making something feel smooth. Here we seem to be embarking more on a “make it look smooth”. (like shiny or polished)

-- Last week I finally got my $*i# together. Unfortunately, it was in my shop, so I will probably never find it again.

Look AND feel smooth. I’m a middle of the road guy. I use mostly oil based poly and use 400 between coats. My main reason for using it is to knock nibs out. My answer to your question is as follows:

You are trying to remove all of the dust nibs and mosquitoes that dried in your finish while you were asleep without disturbing the texture and look of the finish.

The whole sanding between coats to form a mechanical bond idea is a little theoretical to me. I think they’d all stick together whether you sanded or not. The sanding between coats is to keep the finish smooth.

I sand to 180 grit in open grain hardwoods like oak or ash or 220 grit in tight grain woods like maple or cherry. I sand between coats of finish with 400 grit and then rub the final coat with a brown paper bag. Turning that is a different story because mostly you are sanding cross grain so as a minimum I sand to 600 grit. Rick M is spot on.

There are factions here who have recommended scotch-brite pads for this kind of “sanding”.

How do you 600 grit people feel about that? Those that use the 600 grit and like it…have you tried the scotch-brite approach and abandoned it for some reason?

I ask mostly because back when it was too cold to be out in the shop I entertained myself by buying almost everything I read about here on LJ. :) I have a big box of the scotch-brite pads just sitting there waiting for my next project.

-- Last week I finally got my $*i# together. Unfortunately, it was in my shop, so I will probably never find it again.